Editing Planetary core (section)

==Formation==

===Accretion===
Planetary systems form from flattened disks of dust and gas that [[Accretion (astrophysics)|accrete]] rapidly (within thousands of years) into [[planetesimal]]s around 10&nbsp;km in diameter. From here gravity takes over to produce Moon to Mars-sized [[planetary embryo]]s (10<sup>5</sup> – 10<sup>6</sup> years) and these develop into planetary bodies over an additional 10–100 million years.<ref name="Wood, Walter and Jonathan 2006">{{cite journal |last1=Wood |last2=Walter |last3=Jonathan |first1=Bernard J. |first2=Michael J. |first3=Wade |title=Accretion of the Earth and segregation of its core |journal=Nature |volume=441 |issue=7095 |date=June 2006 |pages=825–833 |doi=10.1038/nature04763|pmid=16778882 |bibcode = 2006Natur.441..825W |s2cid=8942975 }}</ref>

Jupiter and Saturn most likely formed around previously existing rocky and/or icy bodies, rendering these previous primordial planets into gas-giant cores.<ref name="Stevenson 1982" /> This is the [[accretion (astrophysics)|planetary core accretion]] model of planet formation.

===Differentiation===
[[Planetary differentiation]] is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components.<ref name="Merriam Webster 2014">{{cite dictionary |dictionary=Merriam Webster |title=differentiation |date=2014 |url=http://www.merriam-webster.com/dictionary/differentiation }}</ref> The [[hafnium-182]]/[[tungsten-182]] isotopic system has a [[half-life]] of 9 million years, and is approximated as an extinct system after 45 million years. [[Hafnium]] is a [[lithophile element]] and [[tungsten]] is [[siderophile element]]. Thus if metal segregation (between the Earth's core and mantle) occurred in under 45 million years, [[silicate]] reservoirs develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated [[chondrite]] material.<ref name="Wood, Walter and Jonathan 2006" /> The observed Hf/W ratios in iron meteorites constrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years.<ref name="Wood, Walter and Jonathan 2006" /> Several factors control segregation of a metal core including the crystallization of [[perovskite]]. Crystallization of perovskite in an early [[magma ocean]] is an [[oxidation]] process and may drive the production and extraction of iron metal from an original silicate melt.

===Core merging and impacts===
Impacts between planet-sized bodies in the early Solar System are important aspects in the formation and growth of planets and planetary cores.

====Earth–Moon system====
The [[giant impact hypothesis]] states that an impact between a theoretical Mars-sized planet [[Theia (hypothetical planet)|Theia]] and the early Earth formed the modern Earth and Moon.<ref name="Halliday and N. 2000">{{cite journal |last1=Halliday |last2=N. |first2=Alex |title=Terrestrial accretion rates and the origin of the Moon |journal=Earth and Planetary Science Letters |publisher=Science |volume=176 |issue=1 |date=February 2000 |pages=17–30 |doi=10.1016/s0012-821x(99)00317-9 |bibcode=2000E&PSL.176...17H}}</ref> During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core.<ref name="Seti Institute 2012">{{Cite web |date=2012 |title=A new Model for the Origin of the Moon |url=http://www.seti.org/node/1458 |url-status=dead |archive-url=https://web.archive.org/web/20121021013405/http://www.seti.org/node/1458 |archive-date=21 October 2012 |publisher=[[SETI Institute]]}}</ref>

====Mars====
Core merging between the proto-Mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years (depending on [[viscosity]] of both cores).<ref name="Monteaux and Arkani-Hamed 2013">{{cite journal |last1=Monteaux |first1=Julien |last2=Arkani-Hamed |first2=Jafar |title=Consequences of giant impacts in early Mars: Core merging and Martian Dynamo evolution |journal=Journal of Geophysical Research: Planets |publisher=AGU Publications |date=November 2013 |pages=84–87 | doi = 10.1002/2013je004587 |bibcode=2014JGRE..119..480M |volume=119|issue=3 |s2cid=41492849 |url=https://hal-clermont-univ.archives-ouvertes.fr/hal-01636075/file/2014.pdf }}</ref>